Production of high grade chemical pulps



UNITED" STATE Patented Sept. 27, I

s PATENT OFFICE GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR'IO BROWN COMPANY, OF BERLIN, NEW'HAMPSHIRE, A CORPORATION OF MAINEPRODUCTION OF HIGH GRADE CHEMICAL PULPS 1T Drawing.

This invention relates to the production of cellulose pulps or kindredraw cellulosic material, beingmore particularly concerned with theso-called chemical pulps in the production of which chemical liquors areemkaline character, e. g.,

indifferent chemical liquors do not act in the same way' upon thenon-fibrous or upon the fibrous constituent of the wood. The acidliquors containing bisulphites are more selective in their action on thepentosans than are :0 the alkaline liquors, but the resulting pulp ineither case has a substantial residue of pen-- tosanS,-amounting to ashigh as 3% to 4% in' the usual commercial sulphite pulps and to as highas 8% to 10% in the usual commercial kraft and soda pulps. It ispossible to remove pentosans from wood pulps by chemical refiningprocesses which include the step of treating the raw pulp under suitableconditions-with solutions of alkalies, but such solutions are notespecialy effective or coni- "plete in their action even overconsiderable periods of time. 3

The production of wood pulps low in pentosan content-and other non-alphacellulose components is highly desirable when such pulps are to be usedin the preparation of cellulose derivatives suchas the nitrate, acetate,and xanthate, and in the manufacture of high grade papers, as it hasbeen found that the" freer the wood fiber is from nonalpha cellulosecomponents including pentosans, the better is the quality, color andstability of its derivatives, and the more near- 1y do papers madetherefrom approach the qualities of new-rag papers.

I have found that if .wood chips, e. g.,.

spruce, or kindred raw cellulosic material is initially treated with asolution of a mineral acid, and more particularly one which is highlyionized in water, and is then cooked at Application filed December 18,1929. Serial No. 415,124. r

elevated temperature and under pressure in a solution of a suitablealkali, for instance in the usual kraft or soda liquors, it is possibleto produce cellulose pulps of high alpha cellulose content and ofexceedingly low pentosan content. The resulting product may have analpha cellulose content of from i 94% to 98% and a pentosan content ofless than 1%, depending upon the particular conditions under which theinitial treatment with acid solution and the subsequent cooking inalkaline liquor are carried out. In bleached condition, the product iseminently suitable for use in the papermaking and derivative industries.One of the major ad-' vantages of the process is that it may becontrolled to produce a product of unusually low solution viscosity,as-some derivatives,

such as the lower nitrocelluloses intended for l lacquer manufacture,must have low solution viscosity. Whereas it is necessary to subjecttheusual fibers or derivatives to special chemical treatments designed tobring about a lowering of solution viscosity, the product of the presentprocess need not requlre such treatment. I do not, however, herein claimthe product, as this subject matter has been described more fully andclaimed in my application Serial No. 435,962, filed March 14, 1930.

, Evidently the use of mineral acid solutions in the initial treatmentcauses a substantially complete hydrolysis of the pentosans into solublesugars and other reaction products which are readily soluble in alkalinecooking liquors. The initial treatment is not in the nature of afiber-liberating operation, sinceeven When sulphurous acid solutions areused, they should be substantially free from bisulphites, which promotefiber liberation by reacting upon and dissolving the ligneous a stronglyacid liquor, it is essential that the I i be effected while the woodparticularly advantageous,

acid solution ers are bound together and are associated with morereactive constituents, which evidently protect the fibers fromdegradation by undergoing reaction themselves. ment may also transformthe alpha cellulose to a form profoundly diiierent from that present inwood pulp, such as sulphite or kraft, for not only may the final productbe of surprisingly low solution viscosity when tested by the usualcuprammoniumcellulose solution standard, but other derivatives, such asthe nitrocelluloses prepared therefrom, have even lower solutionviscosity than similar derivatives prepared from other fibers whosecuprammonium cellulose solution viscosity has been lowered by one ormore special treatments to the same value as that possessed by acuprammonium cellulose solution prepared therefrom.

In the initial treatment ofthe wood chips with an acid solution, variousacids and or acid salts and various conditions oftreatinitial treatmentwith strolgi ment may be used. The acids or acid salts employed,however,should be comparatively strong or highly ionized in a ueous solution andto this end mineral aci s such as sulphuric, sulphurous, nitric, andhydrochloric, or acid salts of, these acids, are suitable, since theyeffect the desired reaction upon the pentosan content of the wood. Theuse of involatile mineral acids such as sulphuric is inasmuch as thereis no liberation of obnoxious gas from the solution even when thetreatment is carried out at boiling temperatures in open tanks. Ifdesired, a sufliciently strong solution of such mineral acids may beused'so that the desired reaction takes place under room temperatureconditions, under which conditions the use of special apparatus andsteam is avoided. For instance, when using concentrated acid solutionsof from 15% to 35% acid content, the treatment may be carried out'underatmospheric pressure condi-. tions of, say, from 20 to 40 C. for aperiod of about one hour, but the use of dilute acid solutions of, say,an acid content of about 1% makes necessary the use of much highertemperatures, for instance, 100 C., or higher, and a periodof treatmentof about two hours.

The acid liquor associated with the wood after the treatmentis of darkbrown color as a result of the solution of organic matter and thehydrolysis of pentosans present in the wood to soluble sugars. When astrong sulphuric} or other acid solution is initially employed,separated from the wood and employed wholly or in part for the treatmentof other raw wood, but if too dilute or contaminated for reuse,its'sulphur content, in the case of sulphurousor sul huric acidsolutions, may

recovered by m xing it with the spent alka- The initial acid treat-v itmay be drained or otherwiseline liquor recovered from the second cookingstep before the latter is subjected to the well-known recovery treatmentpractised in the kraft process? solutions or of niter cake solutions inthe acid treatment of the wood is not only advantageous for thosereasons already glven, but

The use of sulphuric acid because these chemicals are comparativelyinexpensive and furnish a high concentration of hydrogen ion, whichevidently favors the desired hydrolyzing action on the pentosans. Thesechemicals are further advantageous,

in that the sulphur content of the spent acid may be of high sulphidity,may be advanmy process, as it is preferable to use one containing sodiumsulphide, because the resulting pulp has better physical characteristicsand is more easily bleachable.

The acid-treated wood is of much darker' color than the raw wood,usually being of a reddish hue. While it may be readily crushed at thisstage into fragments, nevertheless it is so high in ligneous matter thatit does not lend itself'to ready disintegration into a pulp suitable forconversion into paper or derivatives. The wood is preferably washed freeof residual spent acid liquor before it is subjected to cooking in analkaline liquor which, as previously stated, preferably consists of asolution of sodium hydroxide and sodium sulphide and may beef aconcentra- I tion and composition similar to that used in the kraftprocess. The chips may, for example, be cooked in a liquor containingabout 1.5 1pgunds of alkali per cubic foot, both inthe fo' of causticsoda and sodium sulphite, thealkali compound being calculated as activeNa O. The sulphidity of the liquor may amount to about'40%,whichcorresponds to the sulphidity of a typical kraft liquor.

The smelted compounds, which I tageously employed in-the production ofthe I alkaline cooking liquor for the second step of Thecooking-operation mayfbe carried out in the usual alkaline digester andthe digester contents may be heated to a maximum tempe'rature of from300 to 335 F. in two.

hours, after which the digester. contents may be maintained at thistemperature with a- .of about 100 I corresponding steam pressure poundsfor about two hours. At the end of this cooking operation, the wood hasbeen converted into a pulp, so that the digester may be blown as usualand the pulp washed free of spent 1i uor. The pulp may be'con- 'vertedto a pro uct of high whiteness by the use of hyp ochlorite bleachliquor, but to bleaching operation, it is -in asolution containing 1%.at a suitable stock consistency wlth about 1% chlorine, based on theweight of dry pulp, whereupon it may then be washed, digested causticsoda, based on the weight of dry pulp, and a ain washed. Thereupon itmay be blcache to a high degree of whiteness with only about 2%hypochlorite bleach, based on the weight of dry pulp. In order topreserve the hi h alpha cellulose content of the pulp during t ebleaching in a hypochlorite liquor containing a slight amount of freealkali, such as caustic soda, and to maintain such liquor at atemperature of between and 120 F. In lieu of hypochlorite bleach, otherbleaches, such as permanganates, peroxides, perborates, or the like, maybe employed.

In the initial treatment of the raw cellulosic material with a mineralacid solution, it may be desirable to add a small amount of suitableoxidizing or reducing agent to the solution when the acid itself doesnot act as such, as the presence of such agents often accelerates thedesired-modification in the raw material and, in the case of reducingagents, sometimes improves the papermaking characteristics of thefinished product. For instance, whena solution ofsulphuric acid isemployed, an oxidizing agent such as permanganate may be added thereto,but no such addition need be made to a solution of nitric acid, whichitself is an oxidizing agent.

One ofthe major advantages residing in a process such as described isthat it is possible to produce a product having the desirablecharacteristics mentioned, by the use of comparatively inexpensivechemicals and by rapid treatment in inexpensive apparatus. In producingrefined pulps from wood or kindred raw cellulosic materials, it has heretofore been the practice to repare acraw wood pulp such as sulphite orraft, and then to dissolve non-alpha cellulose components from such pulpby the use of alkaline liquors. Aside from the fact that such a processdoes not result in a product of minimum pentosan content or ofexceedingly low solution viscosity, strong alkalies, such as caustic.soda, are much more expensive than acids such as sulphuric, and it isgenerally necessary to consume, as much, if not more, of such alkali toproduce a given weight of finished pulp.

I claim:

1. A process which comprises treating chipped rawwood with a solution.of sulphuric acid to effect reaction more particularly upon the pentosancontent of the wood, and then cooking the wood at elevated temperatureand under pressure in an alkaline liquor to effect fiber liberation intoa pulp of high preferable to effect alpha cellulose content viscosity.

and low 'solut ion 2. A process which comprises treating chipped rawwood with a hot, dilute solution of sulphuric acid to efiect reactionmore particularly upon the pentosan content of the wood, and thencooking the wood in'an alkaline liquor under pressure to efiec't fiberliberation into a pulp of high alpha cellulose content. and low solutionviscosity.

3. A process which comprises treating chipped raw wood witha solutionofsulphuric acid to effect reaction more particular- 1 ly upon thepentosan content of the wood, washing the wood, and then cooking thewashed wood in an alkaline liquor under pressureto efiect fiberliberation into a pulp of high alpha cellulose viscosity.

4. A process which comprises treating chip ed wood with solution ofniter cake to e ect reaction more particularly upon the pentosan contentof the wood, and then cooking the wood in an fiber liberation.

5. A process which comprises treating chipped wood with a solution ofsulphuric acid to efi'ect reactio:;. more particularly upon the pentosancontent of the wood, and then cooking the Wood to eifect fiberliberation in 3.11: (elkaline liquor containing sodium sul' p l e. I

6. process which comprises chipped raw Wood with a solution phuric acidto effect reaction more particularly upon the entosan content of thewood, and then coo ing the wood at elevated tem-' perature and underpressure in an alkaline liquor containing sodium sulphide to effectfiber liberation into a high grade paper-making pulp of high alphacellulose content and low solution viscosity.

In testimony whereof I have affixed my signature.

' GEORGE A. RICHTER.

treating of sulcontent and low solution alkaline liquor to effect Ill

